The purpose of this investigation is to examine the immunogenicity of liposomal model membranes that have been sensitized by the incorporation of phosphatidylethanolamine (PE) derivatives in which the amino group has been substituted with various haptens. In previous studies (with guinea pigs, rabbits, and mice), we have shown that -depending on the determinant - such liposomes can elicit either a cellular and/or humoral response. (Cellular response was determined by the appearance of delayed reactions upon intradermal challenge with the appropriate test material; humoral response (i.e. antibody formation) by serum hemagglutination titer, binding of radioactive hapten to serum protein, appearance of plaque forming cells in lymphoid tissues). In these liposomes, the determinants (i.e., the N-substituted PE derivatives), are inserted non-covalently in lipid bilayers which function as the carrier. They are thus novel immunogens that differ significantly from conventional immunogen preparations in which haptens are covalently attached to carriers such as proteins; liposomes should therefore be explotied to investigate numerous phenomena that have so far been studied solely with classical hapten-carrier conjugates. They are uniquely suitable for this purpose because epitope density, number and molecular complexity of the determinants, and their physical-chemical environment can be simply regulated by varying the composition of the lipid mixture used to generate the model membranes.